A fiber laser resonator cavity has a reflector at each end of a length of an optical fiber that includes a doped core surrounded by one or more cladding layers. Generally, one reflector is a high reflector, having a reflectivity of close to 100% at the lasing wavelength, and the other reflector serves as the output coupler and typically has a reflectivity between 0.5% and 50% at the lasing wavelength. Reflectors can be formed by simply cleaving or polishing the ends of the fiber near perpendicular to the fiber axis and, if necessary, applying a coating to achieve a desired reflectivity. In most fiber lasers the desired output-coupler reflectivity is in the 1-10% range. The precise value has insignificant impact on the fiber laser performance; therefore, one very inexpensive, robust, and frequently-used option for the output coupler is to utilize the normal-incidence Fresnel reflectivity of the uncoated surface of around 4%, depending upon the refractive index of the fiber core.
In fiber lasers operating at high average power or high peak power, it can be deleterious for the unexpanded beam from the fiber core to impinge directly on the fiber end surface, whether coated or uncoated. Most often, the fiber core has a diameter between 5 microns and 30 microns, with a correspondingly small surface area; therefore at power levels of hundreds of Watts, the power density can approach 1 GW/cm2, which is near the damage threshold of most fiber materials such as silica, particularly in the event of any slight surface imperfection or contamination.
A common technique used to reduce this high power density at a fiber end is to splice a short length of coreless fiber or rod onto the fiber end. Such a short length of fiber or rod is often referred to as a fiber end cap or a beam expander. In operation, a beam emitted from the fiber core, upon entering the end cap, ceases to be confined and expands as it traverses the end cap, exiting through the end cap surface with an increased beam diameter and correspondingly lower power density. The end cap is usually 0.1-5 millimeters long and increases the beam diameter up to 50-300 microns. Since the power density scales as the inverse square of the beam diameter, a 5-10 times increase in diameter will lower the power density by a factor of 25-100 times, greatly improving the reliability of the fiber tip.
In order to operate a fiber laser at high power levels with one or more fiber-end reflectors, either coated or uncoated, it would be desirable to incorporate a fiber end cap into the fiber end in order to reduce the power density. However, it is neither sufficient nor practicable to simply attach a conventional end cap to a reflective flat output surface. Indeed, the reflected light in this case would continue to expand as it passes back through the end cap, and the beam would not be coupled efficiently back into the fiber core.
For example, if for an output coupler end cap, a 10% reflective surface is used with 5% coupling efficiency, then 0.5% of the light incident on the end cap surface will be reflected and fed back into the oscillator, 9.5% of the light will be reflected and lost, and 90% of the light will be transmitted. For typical applications, the coupled fraction of the backreflected light should be at least 4-5%; otherwise the system would have insufficient feedback. Thus, the application of prior art end caps as output couplers for a high power fiber laser is inefficient resulting in a considerable amount of wasted light and poorly controlled lasing due to insufficient feedback.
An external bulk optic mirror can be used as an output coupler in a fiber laser. However, such a mirror has to be aligned with a high precision in an angular, as well as in a linear sense; moreover, the high accuracy of alignment would have to be maintained over the operational temperature range of the laser. A high degree of alignment stability is difficult to achieve, especially in high power lasers which generate large temperature gradients during normal operation. Moreover, when an external output coupler, such as an external concave mirror, is used for providing feedback into the fiber laser, there is an additional interface which the light must pass through in order to reflect off that external coupler and couple back into the fiber. Such an interface would exhibit a potential point of failure due to high optical power densities at the interface as has been explained above. Therefore, it is preferable that an output coupler of a fiber laser has no air gaps, or voids, for the laser light to pass through in its way from the fiber core towards the output coupler reflective surface; ideally, the output coupler would have to be bonded to the fiber directly, such that the laser light does not encounter a significant change of the index of refraction as it travels from the fiber core and towards the output coupler reflective surface.
It is an object of this invention to provide a monolithic end cap that will ensure that a sufficient fraction of the radiation reflected from the output surface of the end cap is coupled back into the fiber core.
It is also an object of the invention to provide an end cap with an efficiency that would couple at least 50% of the backreflected light back into the fiber core. This high coupling efficiency is achieved without having to rely on often difficult and tedious alignment of external optical elements. Prior to this invention, it would have been very difficult to achieve the required precision in transverse, longitudinal, and angular alignment of an external bulk optic mirror to achieve efficient coupling of the reflected light back into the laser core.